This invention relates to crystalline salts of N-[4-[[(2-amino-1,4,5,6,7,8hexahydro-4-oxo-5-methyl-(6S)-, -(6R)- and -(6R,S)-pteridinyl)methyl]amino]benzoyl-L-glutamic acid (hereinafter called salts of 5-methyltetrahydrofolic acid), to the use thereof, and to a method of producing them.
Tetrahydrofolates are predominantly used as 5-formyltetrahydrofolic acid and the salts thereof (leucovorin) or as 5-methyltetrahydrofolic acid and the salts thereof, for the treatment of megaloblastic folic acid anaemia, as an antidote for increasing the compatibility of folic acid antagonists, particularly of aminopterin and methotrexate in cancer therapy (xe2x80x9cantifolate rescuexe2x80x9d), for increasing the therapeutic effect of fluorinated pyrimidines and for the treatment of autoimmune diseases such as psoriasis and rheumatoid arthritis, for increasing the compatibility of certain antiparasitic formulations, for instance trimethoprim-sulfamethoxazole, and for reducing the toxicity of dideazatetrahydrofolates in chemotherapy. 5-methyltetrahydrofolic acid is used in particular as a drug and as a food additive, as a vitamin preparation, for the prevention of neural tube defects, for the treatment of depressive illnesses, and for influencing the homocysteine level.
5-methyltetrahydrofolic acid and salts thereof are extremely unstable, and in particular are highly susceptible to oxidation [see also A. L. Fitzhugh, Pteridines 4 (4), 187-191 (1993) in this respect] and are therefore difficult to produce at a level of purity which is acceptable for a pharmaceutical active ingredient or a food additive.
Various methods, such as excluding oxygen as completely as possible or the addition of antioxidants such as ascorbic acid or reduced L-glutathione, have been employed in order to overcome the instability of 5-methyltetrahydrofolic acid. However, it is scarcely possible completely to exclude oxygen during use, and even then this is only possible at very considerable cost, and the addition of antioxidants is likewise not always possible. Accordingly, it has not been possible hitherto to identify a commercially feasible method which is suitable for the production of salts of 5-methyltetra-hydrofolic acid which are satisfactorily stable and which are of high purity.
Surprisingly, it has now been found that salts of 5-methyltetrahydrofolic acid which exhibit high chemical purity and excellent stability can be obtained by crystallising the corresponding salt from a polar medium after subjecting the solution to thermal treatment at a temperature above 60xc2x0 C. The highly crystalline salts of 5-methyl-tetrahydrofolic acid which are thus obtained are stable at room temperature, practically without-limitation They are suitable as a constituent or as a starting material for the production of drug forms or food additives.
Accordingly, the present invention relates to crystalline salts of 5-methyltetrahydrofolic acid. Alkaline earth salts, particularly the calcium salt, are preferably used as the salts of 5-methyltetrahydrofolic acid for crystallisation. These crystalline salts of 5-methyltetrahydrofolic acid exhibit a purity, which has never been achieved hitherto, of  greater than 98%, together with a stability, with respect to the initial value thereof and which has never been achieved hitherto, of  greater than 98% after storage for 6 months in air at 25xc2x0 C. and 60% relative atmospheric humidity. The crystalline calcium salts of 5-methyl-(6S)-tetrahydrofolic acid exist in four different crystalline modifications (Type I, Type II, Type III and Type IV) and exhibit sharp bands when subjected to X-ray powder diffraction measurements (see Table 1 to Table 4 in this respect). Selected 2 theta values for the different crystalline modifications are 6.5, 13.3, 16.8 and 20.1 (Type I); 5.3, 6.9, 18.7 and 21.1 (Type II); 6.8, 10.2, 15.4 and 22.5 (Type III); and 6.6, 15.9, 20.2 and 22.5 (Type IV). Crystalline calcium salts of 5-methyltetrahydrofolic acid have a content of water of crystallisation of at least 1 equivalent of water per 1 equivalent of 5-methyltetrahydrofolic acid. Thus the Type I modification typically contains xe2x89xa73 equivalents of water, the Type II modification typically contains xe2x89xa62 equivalents water and the Type III and Type IV modifications typically contain xe2x89xa65 equivalents of water.
Other Salts of 5-methyl-(6R)-tetrahydrofolic acid and salts of 5-methyl-(6R,S)-tetrahydrofolic acid can likewise be obtained in highly crystalline form.